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December 2003

Sodium WHAT? If this chemical compound sounds totally unfamiliar, maybe you'll recognize it's common name-chlorine bleach. You may know it as the liquid in that white plastic jug often spotted near the family washing machine. It's added to the laundry to kill germs and to make white clothes whiter. Perhaps you've seen it put to other uses too. Because of its chemical properties, this compound has so many roles in the war against germs, it deserves the title Public Health Champion.

Chlorine bleach, sodium hypochlorite (NaOCl), is a very useful and inexpensive disinfectant. A disinfectant kills germs that can make people sick. Sodium hypochlorite is just one of the common chlorine disinfectants; chlorine gas (Cl2) and solid calcium hypochlorite [Ca(OCl)2] are two others.

Safe Water

Safe drinking water is a basic requirement for good health. In the U.S. we tend to take clean drinking water for granted-it's always there for us. But before chlorine disinfectants like sodium hypochlorite were routinely added to our drinking water beginning about 100 years ago, many people became sick and died of waterborne (meaning "carried in the water") diseases. These diseases were caused by germs infecting people through the simple act of drinking water.

Typhoid fever is a waterborne disease caused by the tiny bacterium Salmonella typhi. This nasty little critter, found in untreated water but visible only through a microscope, causes its victims to suffer with a dangerously high temperature and many other painful symptoms. Before antibiotic drugs were invented to treat bacterial infections, typhoid fever was often fatal. Beginning in 1908, as U.S. water systems began to chlorinate drinking water, typhoid fever and other serious waterborne diseases were practically eliminated, greatly increasing the quality of life for Americans.

Sodium hypochlorite is used in safe food production and preparation. It is added to water in amounts that are known to destroy germs that are associated with raw foods. Sodium hypochlorite solutions disinfect food preparation surfaces, food sorting machinery, containers and instruments of all types involved in producing, transporting and preparing the foods we love to eat.

Medical Uses

Sodium hypochlorite solutions are also used to disinfect many types of surfaces in hospitals, medical labs, doctors' offices and nursing homes to prevent the spread of infection among patients, residents and workers. This is pretty important when you realize that people in hospitals and nursing homes are sick or elderly and therefore unable to fight off infections as well as healthy people can.

The Chemistry of Chlorination

How does sodium hypochlorite defeat germs? The answer is in this chemical reaction:

HOCl, hypochlorous acid, formed when sodium hypochlorite is added to water, penetrates the normally resistant surfaces of microorganisms like Salmonella typhi, destroying them. Whether the acid remains in the form of HOCl or as the ions H+ and OCl- depends on the acidity, or pH of the solution. (Ions were defined in
October's Chlorine Compound of the Month article.)

Follow-Up Questions

1. Natural waters may contain many dissolved chemical elements from the environment. What are some of these elements and how do they change the appearance and taste of water? Define hard water and soft water.

2. The average American uses nearly 50 gallons of water each day. List all the uses of water in an average household. What are the opportunities to conserve water?

3. Drinking water is either drawn from surface water (lakes, reservoirs, rivers) or groundwater (in which case wells are needed). Where does your drinking water come from? How is it treated? (Hint: You may wish to request an annual report from your water utility.)

Science Project Ideas

1. Request a tour of your local water treatment plant and using poster paper, make a diagram, with explanations, of the steps "raw water" is subjected to during treatment.

2. Research some of the microorganisms that cause waterborne diseases in developing countries that lack drinking water treatment.

For a list of previous "Chlorine Compound of the Month" features, click
here.